1. Field of the Invention
The present invention relates to methods and apparatus for the determination of refractive index, including the determination of the absolute value of the refractive index of a sample.
2. Description of the Background Art
Rapid monitoring and detection of ultra small volume samples is in great demand1. Two major areas are environmental monitoring2 and point of care detection3,4. This covers a huge range from drinking water quality5 and food quality6 to determination of glucose7 and urea8. Several detection techniques have the capability to perform fast measurements on small amounts of analyte. Well-established techniques include electrochemistry9,10 mass spectrometry11,12,13 and optical detection14,15. Not all are equally fast since sample preparation often is extensive and requires time-consuming steps such as chemical tagging of molecules16. More or less complex structures are fabricated to guide the samples to the detector17,18. This is done to scale down the required amount of analyte as well as size and cost of the entire system into miniaturized total analysis systems19,20. Simple optical systems are available to perform sensitive measurements on small volumes21. One analytical approach, micro interferometric backscatter detection (MIBD), derives from the observation that coherent light impinging on a cylindrically shaped capillary produces a highly modulated interference pattern. Typically, MIBD analyses reflections from a capillary tube filled with a liquid of which one wants to measure the refractive index. The technique was first used and described by Bornhop et al.22,23. In their work24 the technique has previously been shown capable of measuring changes in refractive index of liquids on the order of 10−7. The MIBD technique is a simple and universal method of detecting refractive index changes in small volumes of liquid. It has been applied to monitor changes in concentrations of solutes25, flow rates26 and temperature27, all conducted in nanoliter volumes. The MIBD scheme and systems of similar geometrical configurations (to that of the MIBD scheme) have previously been modeled by wave theory28,29,30 or optical ray tracing31.
The MIBD technique is based on interference of laser light after it is reflected from different regions in a capillary or like sample container. Suitable methods and apparatus are described in U.S. Pat. No. 5,325,170 and WO-A-01/14858. The reflected or back scattered light is viewed across a range of angles with respect to the laser light path. The reflections generate an interference pattern that moves in relation to such angles upon changing refractive index of the sample. The small angle interference pattern traditionally considered has a repetition frequency in the refractive index space that limits the ability to measure refractive index to refractive index changes causing one such repetition. Such refractive index changes are typically on the order of three decades.